1. Field of the Invention
The present invention relates to a substrate in a reflective liquid crystal display device used as a display of a personal computer and the like and a method for producing the same.
2. Description of the Related Art
In recent years, portable OA (Office Automation) equipment such as personal computers have been developed, and a reduction in a cost of a display device has become an important subject. Such a display device has a structure in which a pair of substrates each having electrodes are provided so as to interpose a display medium having electrooptical characteristics therebetween, and a voltage is applied across the electrodes, whereby a display is performed. Liquid crystal, an electroluminescence material, a plasma material, an electrochromic material, etc. are used for such a display medium. In particular, liquid crystal displays (LCDs) have been most developed for commercial use since they allow a display to be performed with low power consumption.
Considering a display mode and a driving method of a liquid crystal display device, a simple matrix system using a super twisted nematic cell is suitable for the largest cost reduction. However, a high resolution, a high contrast, a multi-scale (multi-color, full-color), and a large viewing angle have been required with the advancement of multi-media information, and a simple matrix system is not sufficient for these purposes.
In view of the above, an active matrix system is proposed, in which a switching element (active element) is provided at each pixel, whereby the number of scanning lines (also, called scanning electrodes) which can be driven is increased. Due to the active matrix system, a high resolution of a display, a high contrast, a multi-scale, and a large viewing angle are being achieved. In an active matrix type liquid crystal display device, pixel electrodes provided in a matrix and scanning lines extending in the vicinity of the pixel electrodes are electrically connected through active elements.
The active elements include two-terminal non-linear elements (Metal-Insulator-Metal; MIM) and three-terminal non-linear elements. Currently used representative active elements are three-terminal thin film transistors (TFTs).
In recent years, there is an increased demand for a further reduction in power consumption. Under this circumstance, reflection type liquid crystal display devices have been extensively developed instead of transmission type liquid crystal display devices which usually require backlights.
A liquid crystal display device having both the reflection type function and the transmission type function is proposed (Japanese Patent Application No. 9-201176). The proposed liquid crystal display device has the following structure: in the case of a dark atmosphere, a display is performed by utilizing light transmitted through a transmission portion from a blacklight; in the case where external light is dark, a display is performed by utilizing light transmitted through a transmission portion from a blacklight and light reflected from a reflection portion; and in the case where external light is bright, a display is performed by utilizing light reflected from a reflection portion.
In order to obtain a bright display in a reflection type liquid crystal display device, it is required to increase the intensity of light scattering in a direction vertical to a display screen, with respect to light incident at any angle. For this purpose, a reflective plate having optimum reflection characteristics needs to be produced. More specifically, a surface of a substrate made of glass or the like is provided with controlled unevenness so as to have optimum reflection characteristics, and a reflective plate having a thin film made of silver or the like thereon needs to be formed on the surface.
According to a practically used method, for example, photosensitive resin is coated onto a substrate, and the photosensitive resin is exposed to light through a light-blocking member having circular light-blocking regions and developed, followed by heat treatment, whereby a plurality of convex portions are formed. Then, an insulating protective film is formed along the shape of the convex portions, and a reflective plate made of a metal thin film is formed on the insulating protective film.
Furthermore, the problem of double reflection due to the influence of the thickness of glass which is caused by forming a reflective plate outside a substrate (on the opposite side of a liquid crystal layer) is overcome by forming a reflective plate inside a substrate so as to overlap the reflective plate on the electrodes (i.e., reflective electrodes).
Needless to say, it is preferable to use a material having a high reflectivity for a reflective electrode in a conventional reflection type liquid crystal display device. In this respect, Ag is optimum. However, Ag is a material having a high diffusion rate to an Si layer, so that problems of diffusion to, and reaction with, an underlying layer are serious.
In contrast, Al is not likely to diffuse to, and react with, an underlying layer, and is widely used for metallization in an integrated circuit. Furthermore, Al has satisfactory characteristics such as etching conditions, so that Al is mostly used for a reflective electrode. Such an Al reflective electrode film is etched by a wet etching method in an etchant containing nitric acid, acetic acid, phosphoric acid, and water.
In the above-mentioned prior art, ITO (indium tin oxide) used for a transparent electrode portion is used for lines for transmitting a video signal and electrodes for connecting a driver for driving liquid crystal so as to prevent them from having a high resistance due to oxidation of the connecting portion during the later step.
In the above-mentioned case of forming a reflective electrode on a substrate, particularly during a mass-production step, it is impossible to partially form an Al film without using a special film forming technique such as mask deposition, so that the Al film is formed over the entire surface of a liquid crystal panel including an ITO portion such as a terminal portion. When the reflective electrode film is subjected to wet etching, the following problem arises.
FIGS. 30A through 30F are cross-sectional views showing a process for forming a connecting electrode in a terminal portion of a scanning line of a conventional liquid crystal display device. FIGS. 31A through 31F are cross-sectional views showing a process for forming a connecting electrode in a terminal portion of a signal line of a conventional liquid crystal display device.
As shown in FIGS. 30A and 31A, in a terminal portion of a scanning line and a signal line, a Ta line which is to be a scanning line 1052 is formed on a substrate, and then, a gate insulating film 1055 is formed.
As shown in FIGS. 30B and 31B, an ITO film 1058 which is to be a signal line and a signal line film 1059 are formed. Thereafter, the metal layers formed during the previous step is patterned, whereby a connecting electrode 1065 is formed as shown in FIG. 30C and a connecting electrode 1066 is formed, leaving only a part of the ITO film 1058, as shown in FIG. 31C.
Then, as shown in FIGS. 30D and 31D, an interlayer insulator 1061 is formed. Thereafter, a reflective electrode 1063 made of Al is formed. During a mass-production step, it is impossible to partially form an Al film, so that the Al film is formed over the entire surface of the substrate. Therefore, as shown in FIGS. 30E and 31E, the reflective electrode 1063 is formed even on the connecting electrodes 1065 and 1066. Under this condition, a resist film 1064 is exposed to light and developed by photolithography.
In general, a thin film contains an order-of-magnitude more lattice defects than a bulk material, and the thin film will have an incomplete crystal structure. Therefore, a number of pinholes and active regions are formed in a reflective electrode film. In the case where such a film is developed, the reflective electrode 1063 is damaged by the use of an alkali-type developer, as shown in FIGS. 30F and 31F, promoting the growth of pinholes and the corrosion of active regions.
As described above, according to the conventional method for forming a connecting electrode, a developer comes into contact with the connecting electrodes 1065 and 1066 made of ITO, as well as the reflective electrode 1063, forming a battery system. Due to the reaction of such a battery system, the connecting electrodes 1065 and 1066 and the reflective electrode 1063 are corroded and dissolved, and this electrolytic corrosion substantially decreases the production yield of an active matrix substrate.
As shown in FIG. 32A, an ITO electrode 1041 and an Al electrode 1040 are formed on a glass substrate 1001. In the case where the Al electrode 1040 is patterned to a predetermined shape under this condition, a photolithography step is utilized as shown in FIG. 32B. However, when light exposure and development are performed for forming a resist film 1005, the Al electrode 1040 is damaged by the use of an alkali-type developer. As a result, the corrosion and growth of active regions and pinholes (hereinafter, referred to as pinholes 1007) shown in FIGS. 32A and 32B are promoted (see FIG. 32C). This brings a developer 1006 into contact with the ITO electrode 1041 and the Al electrode 1040 simultaneously, whereby a battery system schematically shown in FIG. 32D is formed. Due to the reaction of the battery system, the Al electrode 1040 and the ITO electrode 1041 are corroded and dissolved, so that production yield of a TFT and production yield of a reflection type liquid crystal display device are decreased without fail. This development step is described in detail in Sharp Report: No. 44, March, 1990 and the like. Therefore, the description thereof is omitted here.